Hi, Tony. I suggest trying four Big Fat Dots under each speaker, directly on the bricks. A fifth Dot in the middle might be beneficial but is more likely to just aggravate your efforts to have solid footing with the other four Dots. Being free-standing, the Dots are extremely versatile regarding placement, so you can fidget their positions a little this way and that to get good footing. It wouldn't matter if a Dot is partially over an indented groove so long as the majority of the Dot is on flat brick. More important is that you get solid seating of the cabinet on each Dot. Besides adjusting their positions, you can also shim one or more Dots if needed: just tear off a few sheets of Post-It Pad paper, however many needed to achieve the height you want, trim for neat appearance and place under the Dot. Or, a felt pad can be used, some kinds of cardboards make very good shimming material. Most likely you won't have to worry about shimming, because you can just move the Dots around (closer to the edges and corners, a little away from the sides, even in an asymetrical array.
Placing something between the Dots and floor can be beneficial and is an area open to trial/audition. If using something like a platform--hardwood, granite slab, etc. you would then need to isolate between the platform and floor.
Herbie's Audio Lab